1. Field of the Invention
The present invention relates to minimizing variations in the speed of a magnetic tape being drawn, from a payout reel to a take-up reel, past a pickup and/or recording head. More particularly, the invention is directed to tape transports, and especially tape transports of the type which receive magnetic tape cassettes, having an automatic control which maintains, within predetermined limits, a constant linear tape speed intermediate the payout and take-up reels. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
While not limited thereto in its utility, the present invention has been found to be particularly well suited for use in cassette recorders intended for employment as digital magnetic memories in data processing apparatus such as, for example, the recently introduced family of apparatus known in the art as microprocessors. Due to the manner of use, and particularly because a cassette recorder employed as a digital magnetic memory will customarily be started, stopped and reversed with a high degree of frequency, reel-to-reel drives are favored over capstan and pinch roller drives in data processing applications. Belts, pulleys, solenoids, and other components subject to deterioration in use, as are employed in capstan type drives, are not required in reel-to-reel drives and control of the movements of the tape can be accomplished electronically by exercising control over the voltage applied to a drive motor in reel-to-reel drives.
In the interest of maximizing the data capacity of magnetic tape cassettes employed as digital magnetic memories, long term speed variations of the tape at the read and/or write head must be minimized. As is well known, as the tape is fed from the payout or supply reel to the take-up reel of a cassette its linear speed will tend to vary. This variation in linear tape speed results from the decreasing angular velocity of the take-up reel as the tape is wound thereon and the simultaneous continual increase in the angular velocity of the payout reel as its effective diameter decreases. Long term speed variation, in part, determines the data capacity of the cassette and, for digital applications, must be controlled to be within .+-. 5%.
The tape is also subject to short term speed variations or "flutter" such as, for example, caused by irregularities on the tape drive motor pulley and belt or on the tape reels themselves. These short term speed variations, which determine the bit error rate, must also be held within predetermined limits. It is to be noted that the short term speed variations are superimposed on the long term speed variations. A speed control technique, to be effective for digital systems, must take into account this inherent summation of the short and long term speed variations.
A number of techniques have been proposed for effecting tape drive speed control in tape transports designed for digital applications. A summary of the state of the art, and a brief discussion of the deficiencies thereof, may be found in U.S. Pat. No. 3,733,529. U.S. Pat. No. 3,733,529 is exemplary of previous attempts to overcome the deficiencies inherent in the use of either capstan type drives or prerecorded clock tracks to achieve constant linear tape velocity. Approaches such as that exemplified by U.S. Pat. No. 3,733,529 are characterized by complexity from an implementation viewpoint and, as is well known, circuit complexities increase cost and have a deleterious effect upon reliability.